1. Field of the Invention
The present invention relates to an optical pickup apparatus. In particular, the present invention relates to an optical pickup apparatus, which enables to reproduce/record data for different kinds of discs with different thicknesses, using an optical pickup.
2. Description of the Related Art
To keep abreast of a recent trend in data digitalization and high capacity data storage, a recording medium also has been changed from a magnetic tape to a disc. Now, the storage capacity of the disc is even enlarged by increasing recording density for recording data in the disk.
As a result of long, untiring efforts for increasing the recording capacity, a digital video disk (hereinafter, it is abbreviated to DVD) having high recording capacity was first developed from the existing compact disk (hereinafter, it is abbreviated to CD). Before long, a high density (hereinafter, it is abbreviated to HD) disk was introduced. Amazingly, the HD disk could record and reproduce the data at a very high density (i.e., twice of CD), using the existing CD. The emitting wavelength from a laser diode light source is also different for each disk. For example, in case of the CD, the wavelength is equal to 780 nm, and in case of the DVD and the HD, the wavelengths are equal to 650 nm and 405 nm, respectively.
The numerical aperture (NA) of a typically used objective lens in an optical pickup apparatus for reproducing HD is equal to 0.85 and the thickness of its disk is equal to 0.1t. Meanwhile, the numerical aperture of the objective lens in an optical pickup apparatus for reproducing DVD is equal to 0.6 and the thickness of its disk is 0.6t.
As aforementioned, the numerical aperture of the HD use objective lens is 0.85, which is larger than the numerical aperture of the DVD use objective lens, 0.6. Therefore, when the HD use optical pickup apparatus is employed, one realizes that one objective lens is not just sufficient to satisfy the numerical aperture requirement. For this reason, two objective lenses are used instead of one, as shown in FIG. 1, to make the numerical aperture 0.85.
FIG. 1 is a schematic diagram showing a configuration of a HD use optical system in the prior art. As depicted in the drawing, the HD use optical system includes a first objective lens 110, whose incident plane 112 and outgoing plane 114 are aspheric, and a second objective lens 120, whose incident plane 122 is spherical but the outgoing plane 124 is flat. Alternatively, the optical system can be configured by making the incident plane 112 and the outgoing plane 114 of the first objective lens 110, and the incident plane 122 of the second objective lens 120 all aspheric, while making the outgoing plane 124 of the second objective lens 120 flat.
Still as another option, the optical system can be configured by making the incident plane 112 of the first objective lens 110 and the incident plane 122 of the second objective lens 120 aspheric, and making the outgoing plane 114 of the first objective lens 110 and the outgoing plane of the second objective lens 120 flat. Here, the reference numeral 130 can be either HD or DVD.
In short, the HD use optical system can have a variety of configurations by alternating the shape of the first objective lens 110 and the second objective lens 120.
However, if one intends to reproduce DVD using two objective lenses appropriate for the HD use optical system, the difference in thickness between HD and DVD, that is, 0.1 t for HD and 0.6 t for DVD, causes spherical aberration, and as the result thereof, signal degradation occurs.